Last winter was a real killer for the honeybees — and here's why

Mites, diseases, and pesticides are all suspected of contributing to bee colony collapse disorder. The bees are dying at such a fast rate that farmers who rely on bees for pollination are now reserving them five years in advance. NBC's Anne Thompson reports.

Almost a third of America's honeybee colonies bit the dust last winter, according to a bellwether survey of bee health. But the deaths didn't fit the typical pattern for colony collapse disorder, the mysterious malady that wipes out bunches of bees all at once. Instead, researchers suggest that last summer's drought and other common-sense factors were to blame.

The annual survey of beekeepers, conducted by the Bee Informed Partnership and the Apiary Inspectors of America, found that 31.1 percent of the colonies were lost over the winter of 2012-2013. That compares with a loss of 22 percent during the previous winter, which was exceptionally mild. It's also slightly higher than the six-year average of 30.5 percent in colony losses.

The past winter's bee death rate was roughly as high as it was during the winter of 2006-2007 — when colony collapse disorder, or CCD, was at its peak. But this time, most colonies "dwindled away rather than suffering from the sudden onset of CCD," Jeff Pettis, a U.S. Department of Agriculture bee expert who worked on the survey, said in a news release announcing the results.

University of Maryland entomologist Dennis vanEngelsdorp, who directs the Bee Informed Partnership, listed several likely causes for last winter's spike. One prime reason is the drought that swept over the Midwest last year. "When there's a drought, the bees are in poor shape with the food," California beekeeper Randy Oliver told NBC News in March.

Honeybees may have had to rely on irrigated crops rather than wildflowers for their nectar, which could have increased their exposure to pesticides, vanEngelsdorp said. He said last year's rising corn prices led farmers to replace prairie and shrubs with cornfields, further limiting the bees' foraging areas. And for part of the year, beekeepers lacked an effective treatment for Varroa mites, a type of bee parasite that was cited last week as the biggest factor behind the nation's bee die-off.

VanEngelsdorp said all these factors left bee colonies in a weakened state for the tough winter of 2012-2013. He said the beekeepers who took their hives to California in February to pollinate almond trees suffered especially high losses. Nearly 20 percent of those beekeepers said they lost 50 percent or more of their colonies over the winter.

Pettis noted that the survey stopped tracking losses at the end of April. As a result, "the 31 percent figure likely underrepresents the losses, as we saw many weak colonies that were not actually dead," he said.

Beekeepers rebuild their colonies in the spring, so a 31.1 percent loss rate isn't quite as catastrophic as it sounds. Nevertheless, vanEngelsdorp said high winter losses are changing the way commercial beekeeping is done. "All the money you're going to make in honey goes to replacing dead colonies and keeping your colonies alive," he said. "Any money you make [as profit] will be from pollination."

The winter colony loss survey was funded by USDA. The 6,287 U.S. beekeepers who responded to the survey managed nearly 600,000 bee colonies at the start of the survey period, or about 23 percent of the country's estimated 2.6 million colonies. A complete analysis of the survey data will be published later this year.